Changes LoadGlobal, StoreGlobal, LoadNamedProperty, and StoreNamedProperty to accept
the name of variable instead of index into the constant pool entry. Also made
GetConstantPoolEntry as a private function since it is no longer used outside of
BytecodeArrayBuilder.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1546643002

Cr-Commit-Position: refs/heads/master@{#33020}